Portable electronic devices such as Personal Navigation Devices (PND) are increasingly utilized. A PND includes a Global Positioning System (GPS) receiver configured to receive signals from a group of artificial earth satellites called the Global Navigation Satellite System (GNSS). The GPS receiver uses the arrival time of signals from four or more satellites to compute the distance to each satellite. From the relative distances the GPS receiver is able to determine its position, and therefore a user's position, using geometry and trigonometry.
In some cases a vehicle may be equipped with a navigation device integrated with the vehicle's dashboard, and with the vehicle's electronics. In other cases a portable PND may be brought into the vehicle, and may or may not receive power from the vehicle's power supply. The detected position of the GPS receiver may be depicted as a graphical element superimposed over a map on a display to help a user navigate. The graphical element may remain more or less in the same location on the display while the regions displayed on the superimposed map adjust as the vehicle moves, illustrating movement of the user across the map.
A problem sometimes arises when one or more of the signals from the GNSS are dropped. This may happen, for example, if the vehicle goes into a tunnel, or travels adjacent one or more tall buildings, thereby blocking the signal(s) from reaching the receiver. As a result, erroneous display of the position of the vehicle on the map may occur, causing the user to become lost, confused, or miss a turn.
Efforts have been made to provide dead reckoning with PNDs by providing a PND with a built-in accelerometer. This approach has several drawbacks. First, this approach typically only provides dead reckoning for a few seconds at a time. In addition, this approach involves a double integration of the sensed acceleration, which causes accumulated errors in speed and distance calculations.